How is downforce measured (the exact amount of of produced at speed..)?
Andrew- It is usually measured just in pounds on the front and rear axles. Pretty much need a wind tunnel to get accurate results, although CFD can give you good approximations if it was grounded using real data. CFD models without real data around which to base them can give some squirrelly results. Taz Terry Phillips
You can measure it by using the same linear potentiometers that are used on any data acq. It can actually be extrapolated very easily. jay
Jay- You are correct. If you want to put orange wires on the car and measure suspension compression, it is not that difficult. You could actually do the same thing with a laser measurement of ride height while at speed. Either set of numbers is easy to convert to pounds, since you know the spring parameters. Taz Terry Phillips
The difficulty in calculating downforce (or anything aerodynamic) is that you don't have one single formula, but have to use special numerical techniques with special sets of formulas to calculate your result.
You find the linear position of the car sitting on a dead flat surface (ride hieght null point) You find out the spring rates front and rear (WSM) You determine the spring rate to wheel rate via suspension geometry (geometry and trig) Then you take the car up to speed and measure the change in ride height versus speed on a dead flat road surface. Then you back calculate via wheel rate out how much downforce (or upforce) existed Alternately you can add weights to the front and rear of the car until the ride height matches that at speed.
Terry, So in a wind tunnel, they will measure how much "heavier" the car gets on the front and rear axles?
Although these factors affect downforce, they don't affect the relationship between downforce and subsequent ride height. 200# of down force has the same effect on ride height, regardless of the conditions that created it.
Andrew- In a wind tunnel, any of the methods mentioned above becomes very simple. A moving road wind tunnel like Ferrari uses makes it very simple to measure ride height directly or to measure spring compression directly without having to use orange wires. Just hook up sensors, measure directly, and record the data through a wire (or wireless) connection between the sensor and your data recording system. Either, or both for accuracy, gives you almost a direct reading of downforce if your system is calibrated for that particular suspension system. Taz Terry Phillips
First, I stand on the scale and take a reading without any wind. Then, I turn on one of those big ass shop fans and see what that scale reads. Depending on how I hold my hands and comb my hair, I'm up to 14lbs of down force. GT
Yup, in theory. If you've ever tried to do this in practice, you find out very quickly that there's no such thing as a dead level road. Even on the flattest road you can find, the car is still weaving and bobbing around on the springs, so, what you end up with it a whole pile of data points cross referenced to timing marks, and then you have to run statistical formulae to then try to build a model for speed v. average spring measurements. It ain't as easy as it sounds. Another way to measure down force very precisely is to take your MG out on a straight road and drive it as fast as it will go. Pretty soon you'll feel the front end get so light that it's clear the car wants to fly. At the precise speed at which the front wheels leave the ground, then you've achieved exactly the (negative) downforce equivalent to the front axle weight of the car.
Of course, but you have to know the conditions for the number to have any real meaning. You could generate 1500# of downforce but your wing stalls at 50MPH after which maybe you even generate lift.
Load cells under the spring perches, shock pots and laser ride height front and rear along with a true air speed sensor (pitot tube) can supply the information you need. Mitch, if only the track were flat! <grin>
Strain Rosettes; 125UY CEA 120, 350 0.125 3.18 Three-element 60° delta single-plane rosette. Matrix size: 0.50L x 0.44W in. (12.7L x 11.2W mm) Placed on the flexible CF wing standards, will give real world downforce mapping. After they are calibrated in a wind tunnel.. Edwardo
Simple. Drive at high speed on an upside down road and then slow down. Note the speed when you fall off. Downforce = weight of car at X speed. Its logrithmic in either direction from there.
Many years ago when I worked on 962s we went to the Michelin test track which was equipped with a built in scale. The cars weight was measured as it went across. We made frt and rear wing adjustment to collect the data needed. Karl
Generating aeromaps http://www.compsystems.com.au/index.php?option=com_content&view=article&id=97:aeromapping&catid=7:datatech&Itemid=6
It won't give you numbers but in practical racing terms you can tell by making a change on one end of the car and see how it effects the handling of the car in high speed turns. An increase in downforce in the rear will make the car push, an increase in the front downforce will make the car push etc. You can really feel the difference on the track when you make a change at one end of the car. Then you have to decide if the increase in downforce is worth the aerodynamic drag that usually comes the downforce. The car will stick like glue in the turns but you'll feel like your dragging a chute down the straights if the wings are turned up too high. I cranked the wings all the way up one time on a F2000 car and it bottomed out all the way down the straights. The biggest laugh I get is from these tuner car guys that put these huge wings on the back of the street cars. Most of these cars probably push to begin with, adding the huge wing on the back probably makes them push like a pig at high speed if the wing actually gets some clean air.
